1. Field of the Invention
The invention relates to a fastening structure of a power unit for integrally joining an internal combustion engine and a transmission by fastening them together with bolts to form the power unit.
2. Description of the Related Art
Generally, in power units that are mounted in vehicles, such as motor vehicles or the like, an internal combustion engine and a transmission are fastened together with bolts. For example, in a power unit described in Japanese Utility Model Application Publication No. 3-43160 (JP-U-3-43160), an internal combustion engine and a transmission are integrally coupled by fastening them together with a plurality of bolts while a junction surface formed on an end of the cylinder block of the engine and a flange-shape junction surface formed on the case of the transmission are placed in contact with each other.
In a power unit in which an internal combustion engine and a transmission are fastened together with a plurality of bolts in the foregoing manner, a gap is sometimes formed between the junction surface of the internal combustion engine and the junction surface of the transmission due to variations in the working process of the junction surfaces, the mounting error caused when bolts are fastened, the deformation of the junction surfaces caused by thermal expansion occurring during operation of the engine, etc. Incidentally, such a gap tends to be formed in a portion of the junction surfaces in which the fastening force exerted by a bolt acts less easily or less certainly, that is, in a so-called bolt span portion between a bolt-fastened portion and another bolt-fastened portion.
If such a gap is formed between the junction surfaces of the internal combustion engine and the transmission, the junction surfaces vibrate in a portion where the gap is formed, when vibration attributed to a mesh transmission error of a gear train of the transmission or the like is transmitted to the junction surfaces through the case of the transmission.
Then, if the frequency of the vibration transmitted to the junction surfaces from the transmission is close to the eigen frequency of a portion of the junction surfaces where the gap is formed, resonance may possibly occur in the portion where the gap is formed, so that large vibration or noise may occur.